From your error message, it seems that Current_List is an empty string and not a list, and that's what caused the error.
We need your full code to understand why. Please right-click an empty space in the Blocks editor, select Download Blocks as Image, and upload the image here. What might help is replacing the input of initialize global Current_List to an empty list if the input is a string, but this is only an assumption.
Upon initialization, I am getting the start value from the Schedule screen (empty value if a new or no schedule is available or a start value of the selected schedule). Setting the global Selected_List to start value.
I then set the Current_List to TinyDB Tag global Selected_List with value "" if not there.
This was the problem, I changed with "Create empty list"
Nice job. Yes, a lot of people forget replacing the default valueIfTagNotThere text block, which sets the variable to a string. I've set your answer as the solution.
A lot of times, the hardest thing is to find our own mistakes ...
Well, for me currently, the hardest thing is sorting the list then displaying it in list view without the brackets and symbols. So, I'm on to that now. Thanks for the response though.
If you want to create a ListView with detail text, you must change ListViewLayout or it will not work. You might want to consider setting ListViewLayout to MainText, DetailText (Horizontal).
As for sorting the list, this code seems to be working.